Method and apparatus for ignition of crossed field switching device for use in a hvdc circuit breaker

ABSTRACT

Ignition method and apparatus for crossed-field switching device for onswitching the device when a high voltage is applied thereacross, comprising applying a plasma puff between the electrodes to initiate arc discharge therebetween, followed by resonantly reducing the current to zero to extinguish the arc and permit initiation of glow mode discharge after the current zero.

United States Patent [191 [111 3,714,510 Hofmann Jan. 30, 1973 [54]METHOD AND APPARATUS FOR [56] References Cited IGNITION OF CROSSED FIELDSWITCHING DEVICE FOR USE IN A STATES PATENTS HVDC CIRCUIT BREAKER3,290,542 12/1966, Lafferty ..337/15 3,534,226 l0/l970 Lian ...200/l44 R[75] Holman L08 3,356,897 l2/l967 Barr ..313 |s0 geles, Calif. [73]Assignee: Hughes Aircraft Company, Culver r mary inerJ. D- Miller City,Calif. Assistant ExaminerHarvey Fendelman Attorney-W. H. MacAllister,Jr. and Allen A. Dicke, [22] Filed: March 9,1971 Jr.

[2]] Appl. No.: 122,397 [57] ABSTRACT Ignition method and apparatus forcrossed-field [52] US switching device for onswitching the device when aInt Cl H05! 7/00 5 17/26 high voltage is applied thereacross, comprisingapply- [58] Field iiii 7667144 A 14 6 A- ji37/ 5- g aplasma puff betweenthe electrodes to initiate arc discharge therebetween, followed byresonantly reducing the current to zero to extinguish the arc and permitinitiation of glow mode discharge after the current zero.

I l e i l l l l l l l l J L METHOD AND APPARATUS FOR IGNITION OF CROSSEDFIELD SWITCHING DEVICE FOR USE IN A I-IVDC CIRCUIT BREAKER BACKGROUNDThis invention is directed to a method and apparatus for the ignition ofa crossed-field switching device for onswitching high voltage DCcurrent.

Crossed-field switching devices have been known for many years aslaboratory curiosities, as shown in Penning U.S. Pat. No. 2,182,736; andas simple, undeveloped low power devices, as shown in Boucher U.S. Pat.Nos. 3,215,893 and 3,215,939 and Wasa 3,405,300.

Only recently, however, has the utility of such a switching device inhigh voltage, high current DC applications been recognized, becausecareful design is necessary for employment of the crossed-field deviceconcept in high current and high voltage situations. Examples of suchuse are found in Kenneth T. Lian U.S. Pat. No. 3,534,226, and Gunter A.G. Hofmann and Ronald C. Knechtli U.S. Pat. .No. 3,538,960. As describedhereinafter, there is a problem of onswitching a crossed-field switchingdevice when the device is in a non-conductive condition and a highvoltage is applied to its electrodes. The method and apparatus of thisinvention permit the ignition or onswitching of such a crossed-fielddevice under such conditions. Thus, the device can be convenientlyemployed in high voltage DC circuit breaker situations.

SUMMARY In order to aid in the understanding of this invention, it canbe stated in essentially summary form that it is directed to a methodand apparatus for ignition of crossed-field switching devices foronswitching high voltage DC current. The method and apparatus comprise aplasma puffer for introducing a puff of plasma between the electrodes ofthe crossed-field device when the crossed-field device has a highvoltage applied across the electrodes and a magnetic field of sufficientstrength to permit sustained conduction under some interelectrodepotential conditions. The plasma puff causes interelectrode arcdischarge to reduce the interelectrode potential. The method andapparatus further include a resonant circuit which causes aninterelectrodecurrent zero to extinguish the are after the potential isthus reduced. After the current zero, the interelectrode potential risescomparatively slowly to permit conduction in the glow mode.

Accordingly, it is an object of this invention to provide a method forignition of crossed-field switching devices for use in high voltage DCcircuit breakers, the method including initiating an arc dischargefollowed by a current zero to extinguish the arc and followed by asufficiently slow interelectrode potential rise to permit initiation ofdischarge in a glow mode. It is another object to provide an apparatusfor the ignition of a crossed-field tube which includes aninterelectrode plasma puffer and a resonant circuit which reduces theinterelectrode current to zero subsequent to interelectrode arcing.It'is still another object to provide an economic and convenient meansand method which rapidly onswitches a crossed-field switching devicehaving a high potential applied thereto.

Other objects and advantages of this invention will become apparent froma study of the following portion of this specification, the claims, andthe attached drawings.

BRIEF DESCRIPTION OF tHE DRAWINGS FIG. 1 is an electrical schematicdiagram of a power system which includes a circuit breaker employing theonswitching method and apparatus of this invention.

FIG. 2 is a perspective view, with parts broken away and partly taken insection, of a crossed-field switch device, in accordance with the methodand apparatus of this invention.

FIG. 3 is a graph showing the conductivity conditions of a crossed-fieldswitching device relating interelectrode voltage to magnetic fieldstrength.

DESCRIPTION Referring to FIG. 1, the DC power which is to be switched bythe circuit breaker employing the crossedfield switch device ignitionmethod and apparatus of this invention is conventionally derived at apower source 10 which delivers power to an AC generator 12. Generator 12delivers its output to transformer 14 by which the voltage is raised tosuitable transmission line voltage. From the transformer, the power isrectified by rectifier l6. Rectifier 16 has positive and negative outputlines 18 and 20 respectively. Inductance 22, connected in one of thelines and capacitance 24, connected between the lines, serve asconventional DC filtering and smoothing equipment. They are preferablyconnected at the output of the rectifier, as shown. In certaincircumstances, the reactance of the transmission system may besufficient to provide adequate smoothing for economic powertransmission.

Circuit breaker 26 is serially connected in line 18 between therectifier l6 and transmission system 30, while an identical circuitbreaker 28 is connected in line 20 therebetween. Bipolar circuitbreakers are thus provided, because of the high voltages in theexemplary embodiment of employment of the circuit breaker. In

lower voltage systems, only one circuit breaker might be necessary.

In high voltage DC systems, it is customary to have a line potentialsuch that one line is above ground potential, while the other is below.This equalizes the amount of transmission line insulation between thetwo lines and ground. For this reason, the two circuit breakers 26 and28 are required, one in each line. Either one of the lines, through thetransmission system or at the load, may fault to each other, as byexemplary fault switch 32, or can fault to ground. Thus, independentline protection is necessary, should a fault to ground occur, ratherthan the interline fault indicated at 32. However, in either type offault, a circuit breaker is necessary.

Each of the circuit breakers 26 and 28 has conventional fault detectionequipment associated therewith, as well as conventional programmingcircuitry to operate the circuit breaker through its breaking cycle.Thus, conventional fault detection and circuit breaker operatingequipment is included.

Load 34 is connected at the output of transmission line 30. Switch 32schematically represents a potential electrical fault across the lineson the load side of the circuit breakers. The fault can occur anywheretherealong, or between a line and ground.

Circuit breaker 26 comprises the line switch 36 and impedance-increasingmeans 38. Line switch. 36 may have a current transfer circuit inassociation therewith, as shown in a M. A. Lutz and W. F. Long patentapplication Ser. No. 122,395, filed Mar. 9, l97l, entitled CurrentTransfer Circuit as Part of a High Voltage DC Circuit Breaker, filedconcurrently herewith, (PD-70229), the entire disclosure of which isincorporated herein by this reference. Alternatively, any suitable lineswitch 36 which produces a sufficiently high voltage drop thereacrossduring opening to transfer the current into the impedance-increasingportion 38 can be employed. Thus, a simple switch 36 is illustrated,which switch may be a conventional circuit breaker.

The impedance-increasing means 38 is connected between breaker buses 40and 42. Before the impedance is increased, it is necessary that thiscurrent be transferred from the line switch 36, in accordance with theconcept discussed above. This current transfer is aided by crossed-fieldswitch device 44, which is directly connected between buses 40 and 42.Thus, during the time of opening of line switch 36 and the currenttransfer, crossed-field switch device 44 is in conductive condition sothat, when the voltage across the buses 40 and 42 rises to a sufficientpotential to permit glow discharge conduction of the crossed-fieldswitch device 44, it begins conducting and the potential between buses40 and 42 is clamped at the voltage drop of device 44. In devices of thetype under consideration, the voltage drop at high current is about 1kilovolt. This fairly low potential permits the current to be fullytransferred from the line switch 36 so that the line switch can fullyopen, deionize, and be in condition to hold off the surge potential.After line switch 36 has reached this condition, crossed-field switchdevice 44 can be turned off. Thereupon, the impedance can be increasedby the cyclic switching impedance-increasing means described below. I

Surge capacitor 46 and its energy-absorbing resistor 48 are connectedbetween breaker buses 40 and 42 so that, upon opening of the variousswitches, voltage surge peaks are reduced to tolerable limits. Capacitor24 also enters into this effect.

The impedance-increasing circuit selected as an example for employmentof the crossed-field switch device 100, which is equipped foronswitching with a high potential applied thereto, is the cyclicimpedanceincreasing circuit illustrated in Wolfgang Knauer patentapplication, Ser. No. 122,396, filed Mar. 9, 1971, filed concurrentlyherewith, entitled Impedance-lncreasing Method and Apparatus as Part ofa High Voltage DC Circuit Breaker, (PD-69238). Any of the circuitsdescribed in that application can be the impedance-increasing circuit.

Other impedance-increasing circuits can employ the method and apparatusof this invention, the teaching of this invention being the onswitchingof a crossed-field device having voltage applied thereto. Thus, theimpedance-increasing means of K. T. Lian and W. F. Long patentapplication, Ser. No. 45,147, filed June 10, 1970, entitled ConsecutiveCrowbar Circuit Breaker and M. A. Lutz patent application, Ser. No.45,460, filed June ll, 1970, now US. Pat. No. 3,61 1,031, for SeriesSequential Circuit Breaker are pertinent. Each of these disclosures isincorporated herein in its entirety by this reference so that theseveral species of impedance-increasing circuits described therein arewithin the scope of this application.

These circuits are also useful with the switch and method of thisinvention. To illustrate a manner of employment of the invention, acyclic switching circuit is illustrated as the impedance-increasingmeans 38. Serially-connected with the crossed-filed switch device 100 isenergy-absorbing resistor 50. Crossed-field switch device 100 isswitched on and off with increasing offperiods so that the time averageccircuit impedance is increased until the switch can remain open.

In order to aid onswitching, as part of the apparatus and in accordancewith the method of this invention, sufficient capacitance must beconnected across crossed-field switch device 100 and, in associationwith the capacitance, there must be sufficient inductance to resonantlydrive the current to zero. Since the capacitor 46 is connected in serieswith its resistance 48, it normally is not sufficiently close-coupled todevice 100 to accomplish this result. Thus, capacitor 52 is connected inseries with inductance 54 and this series connection is paralleledaround the device 100. Normally, the inductance required is quite smallso that the capacitor leads provide adequate inductance.

To further illustrate the apparatus of this invention, FIG. 2 shows acrossed-field device 100 equipped for ignition, in accordance with themethod and apparatus of this invention.

Referring to FIG. 2, the crossed-field switch device comprises housing102 which is carried upon bottom flange 104. Bottom flange 104 is, inturn, mounted upon base flange 106 and they are secured together toprovide a tight seal. Base flange 106 stands upon foot 108 forsupporting the switch device structure. Furthermore, foot 108 can act asa vacuum connection for drawing a suitable vacuum on the interior ofhousing 102 and then letting into the housing the desired gas (e.g.,hydrogen, including its isotope deuterium) at the required pressure.Housing 102, together with bottom flange 104, serves as a suitablevacuum tight envelope.

Cathode 110 is in the form of a cylindrical tube. It is spaced inwardlyfrom housing 102. Cathode 110 has a lower cap 112 by which it issupported from base flange 104 by means of standoff 114. Lower cap 112does not need to effect closure, but simply provides mechanical supportfor the cathode and reduces plasma end losses. By this construction, theentire cathode can be withdrawn through the large opening in bottomflange 104 when the flanges are separated for inspection and service ofthe cathode and inspection and service of the interior of housing 102.Cathode 110 is metallic and can be made of stainless steel. The cathodeis connected to the foot 108, such as by a metallic strip. Thus, foot108 provides one of the electrical connections to the switching device100. Cathode 110 may have an axial slot to prevent the circumferentialcirculation of current during switching transients, when the axialmagnetic field changes with time.

Anode 116 is of cylindrical tubular construction and is positionedconcentrically with cathode 110 to provide a radial space therebetweenhaving the dimension d. The radial space d is substantially equal at allfacing positions of the anode and cathode. Housing 102 has a top cap 118upon which anode 116 is positioned. The anode is maintained in positionby employing anode cap 120 which is secured to the cylindrical anode 116and, in turn, carries mounting stud 122. Mounting stud 122 provides bothmechanical support by being secured to housing cap 118 and provideselectrical continuity through the cap by electrical connector 124.Preferably, anode cap 120 is spaced below top cap 1 l8 and connector 124passes through insulative mounting stud 122 so that connector 124 andthe entire anode are electrically separated from the housing.Alternatively, top cap 118 can be of insulative material.

Anode 116 may be perforated so that the interior space thereof serves asa gas volume to supply gas to the interelectrode space. Furthermore, gassupply means can be provided interiorly of the anode to supply gas as itis consumed by a glow discharge in the interelectrode space. Both ofthese concepts are taught in Hofmann and Knechtli U.S. Pat. No.3,558,960. The maintenance of interelectrode space gas pressure. isdiscussed in more detail in that patent.

Magnet 126 is positioned on the exterior of housing 30 in such a manneras to provide magnetic lines of force in the interelectrode space whichare substantially parallel to the axis of the electrodes of switchingdevice 100 over at least part of the electrode length. Magnet 126 isillustrated as being an electromagnet and such is preferred, so that themagnetic field can readily be switched on and off. The power supply tomagnet 126 is preferably of such nature as to provide for rapid turnonand off of the field. Its strength is such as to provide a field between50 and 150 Gauss; 70 Gauss was found to be a preferred value for thedimensions given below used in our experiments to date, considering theturnon and turnoff effects, as well as magnet power consumption.

Once a glow discharge is established and current isflowing,,offswitching is accomplished by reducing the magnetic fieldstrength to a point where cascading ionization cannot continue. Thus,conduction ceases. This is explained in considerably more detail in thel-lofmann and Knechtli U.S. Pat. No. 3,558,960, mentioned above.

However, a problem occurs in onswitching of such a switching device.FIG. 3 illustrates the conductive region of a crossed-field device of anature discussed, within the hatched area. When the switching device isnonconductive, the circuit voltage is applied across the interelectrodespace. The illustrated example is a device which switches l00 kilovoltsand, with a 70 gauss magnetic field, the device is in a state indicatedat point A. To render this device conductive, without any other ignitionmeans, the magnetic field would have to be increased so that theoperating point would finally reach the hatched area and, with the 100kilovolts applied to the interelectrode space, a field of nearly 500gauss would be required to initiate cascading ionization.

To overcome this, plasma puffer 128 is secured on device 100 and ispositioned to discharge plasma into the interelectrode space. Plasmapuffer 128 is described in detail in Lafferty U.S. Pat. No. 3,290,542,the entire disclosure of which is incorporated herein by this reference.

When the switching device 100 is in the state indicated at point A inFIG. 3, operation of the plasma puffer 128 places a conductive plasma inthe interelectrode space. This conductive plasma initiates a metallicarc discharge between the electrodes. The electrodes are specifically ofsuch material, such as molybdenum, to permit an appropriate metallic arcdischarge. Considering the circuit of FIG. 1, the capacitor dischargesthrough the switch device, and the circuit inductance is sufficient tocause resonance with the capacitor to cause a current zero in theswitching device. With such current zero, the arc discharge quicklyextinguishes, but now the voltage is reversed. As the circuit brings thevoltage towards zero, the rate of voltage rise being limited by thecapacitance in the circuit, the operating point passes through the toeof the hatched portion of the curve of FIG. 3, moving upward along thegauss line sufficiently slow that a Penning-type of glow discharge isinitiated in the switching device. Once this discharge is established,the device is conducting and the interelectrode voltage is clamped atthe voltage drop of the conducting device.

In a particular example of this physical configuration of the switchingdevice of FIG. 2, the interelectrode radial distance is about 15millimeters, with an anode diameter of millimeters and axial length of300 millimeters. Normal gas pressure in the interelectrode space isabout 0.04 millimeter of mercury. Hydrogen is one possible gas. Withsuch dimensions, the switching device is capable of offswitching DCloads of 1,000 amperes and holdoff 25 kilovolts with recovery time inthe order of about 25 microseconds.

Going through the operation, assuming that the crossed-field device isnonconducting and the potential across the buses 40 and 42 is such thatcapacitor 54 is charged up to voltage of V,,, which is the 100 kilovoltsillustrated at point A in FIG. 3, when the plasma gun 128 injects plasmainto the interelectrode space and initiates breakdown into metallic areconduction, the capacitor 52 discharges through this path. Currentoscillation starts with a frequency given by the capacitance 52 andinductance 54. In view of the desired high frequency, this inductanceshould be very small and, as indicated above, can be the inductance ofthe capacitor leads connecting the capacitor in parallel to the device100.

At the same time, the main current 40 o/ so starts the flow through thedevice and is sumperimposed upon the oscillating current I Afterone-half oscillation, the oscillating current 1,, flows opposite themain current 1 If I is larger than 1 or R 2 VL/C, which is easy toachieve, then the total current through the device goes to zero at timet At this moment, the arc extinguishes and the tube goes into anonconducting state. The ca acitor 52 is left with the reverse voltage VV, V l L/CR'.

The main current now flows into capacitor 52 and drives the voltage ofthe capacitor to zero. This rate of voltage change is slow, compared tothe capacitor voltage change due to the resonant oscillation. Thecapacitor voltage V will be zero at time The rate of change of thevoltage on the capacitor 52, which is the interelectrode potential ofthe device 100 near zero, is

These are favorable conditions for ionization breakdown in theinterelectrode space in the device 100 into the glow mode. The voltageacross the tube rises from zero at a moderate rate, in the order of lkilovolt per microsecond, towards the breakdown voltage of a few hundredvolts. The voltage drop during conduction is very close to the breakdownvoltage so that the capacitor doesnt feed much current into the tubewhen breakdown into the glow mode occurs. Additionally, the time to formthe glow discharge is short enough to allow breakdown during the timethe rising tube voltage is in the breakdown region illustrated in FIG.3. Now that the conduction is in the glow discharge mode, offswitchingcan be controlled by the magnetic field.

The method for initiating switching in a crossed-field device thuscomprises injecting plasma into the interelectrode space of acrossed-field switching device which has sufficient magnetic fieldapplied to the interelectrode space to permit conduction at a lowervoltage when a higher voltage is applied to the interelectrode space toinitiate a metallic arc discharge between the electrodes. Thisinitiation of arcing is followed by resonantly reducing theinterelectrode current to zero to reduce the interelectrode voltage substantially to zero and to extinguish the metallic arc, followed bypermitting the voltage to rise sufficiently slowly to permitinterelectrode current flow in the glow discharge mode.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art.

What is claimed is:

1. An apparatus 'for ignition of a crossed-field switching device toturn on the crossed-field switching device when a high potential isapplied to the crossedfield switching device, said crossed-fieldswitching device comprising:

concentric anode and cathode electrodes having an annular spacetherebetween, magnetic field means for applying a substantially axialmagnetic field to the interelectrode space and a gas within theinterelectrode space so that, during conduction of the crossed-fieldswitch device, current can pass between said electrodes in the glowdischarge mode, the improvement comprising:

a plasma injector positioned adjacent said electrodes for injectingplasma into the interelectrode space and resonant means connected tosaid electrode so that the injection of plasma into the interelectrodespace causes interelectrode metallic arc discharge to reduce thepotential between said electrodes and said resonance means induces acurrent zero to extinguish the metallic arc mode discharge and permitinitiation of glow mode discharge.

2. The apparatus of claim 1 wherein said switching device is part of .acircuit breaker which is connected between an electric current generator and a load.

3. The apparatus of clai 2 wherein said resonant means is a seriallyconnected capacitor and inductor connected in parallel to saidcrossed-field switching device.

4. The apparatus of claim 1 wherein said resonant means is a seriallyconnected capacitor and inductor connected in parallel to saidcrossed-field switching device. i

5. The method of igniting a crossed-field switching device which has apotential applied thereacross of sufficient magnitude to maintain thecrossed-field device in the nonconducting state for the particularmagnetic field applied thereto comprising the steps of:

injecting plasma into the interelectrode space to cause metal arc modeconduction between the electrodes to reduce the potential between theelectrodes to near zero;

resonantly inducing a current zero in the interelectrode current,followed by:

permitting the interelectrode voltage to rise to pass interelectrodecurrent in a glow discharge regime so that current is transmittedthrough the interelectrode space of the device in the glow dischargemode.

1. An apparatus for ignition of a crossed-field switching device to turnon the crossed-field switching device when a high potential is appliedto the crossed-field switching device, said crossed-field switchingdevice comprising: concentric anode and cathode electrodes having anannular space therebetween, magnetic field means for applying asubstantially axial magnetic field to the interelectrode space and a gaswithin the interelectrode space so that, during conduction of thecrossed-field switch device, current can pass between said electrodes inthe glow discharge mode, the improvement comprising: a plasma injectorpositioned adjacent said electrodes for injecting plasma into theinterelectrode space and resonant means connected to said elecTrode sothat the injection of plasma into the interelectrode space causesinterelectrode metallic arc discharge to reduce the potential betweensaid electrodes and said resonance means induces a current zero toextinguish the metallic arc mode discharge and permit initiation of glowmode discharge.
 1. An apparatus for ignition of a crossed-fieldswitching device to turn on the crossed-field switching device when ahigh potential is applied to the crossed-field switching device, saidcrossed-field switching device comprising: concentric anode and cathodeelectrodes having an annular space therebetween, magnetic field meansfor applying a substantially axial magnetic field to the interelectrodespace and a gas within the interelectrode space so that, duringconduction of the crossed-field switch device, current can pass betweensaid electrodes in the glow discharge mode, the improvement comprising:a plasma injector positioned adjacent said electrodes for injectingplasma into the interelectrode space and resonant means connected tosaid elecTrode so that the injection of plasma into the interelectrodespace causes interelectrode metallic arc discharge to reduce thepotential between said electrodes and said resonance means induces acurrent zero to extinguish the metallic arc mode discharge and permitinitiation of glow mode discharge.
 2. The apparatus of claim 1 whereinsaid switching device is part of a circuit breaker which is connectedbetween an electric current generator and a load.
 3. The apparatus ofclaim 2 wherein said resonant means is a serially connected capacitorand inductor connected in parallel to said crossed-field switchingdevice.
 4. The apparatus of claim 1 wherein said resonant means is aserially connected capacitor and inductor connected in parallel to saidcrossed-field switching device.